[sensors] Add more complete examples to the Feature Detection section

anssiko has just created a new issue for https://github.com/w3c/sensors:

== Add more complete examples to the Feature Detection section ==
I'd be good if the spec would add more complete feature detection examples to https://w3c.github.io/sensors/#feature-detection along the lines of the examples provided by @cvan in https://github.com/googlevr/cardboard-vr-display/issues/10#issuecomment-355491381

We should expect many web developers to copy & paste example code from the spec and as such these examples should be more complete.

Please view or discuss this issue at https://github.com/w3c/sensors/issues/339 using your GitHub account